The present invention generally relates to methods and apparatuses for preventing damage to IO devices due to over voltage at pin, and more specifically relates to a method and apparatus for preventing damage to IO devices by band-limiting and source-following and using a comparator to signal a relatively small over-voltage condition on the pad.
Chips are often subject to over-voltage conditions at their input/outputs (IO""s). A common method of protecting the IO""s of a chip is to detect large over-voltage conditions, such as in the range of a thick-gate MOS device""s Vt (i.e., 0.5-0.7 volt). Oftentimes, this amount of protection (i.e., being able to detect a 0.5-0.7 volt over-voltage condition) is not enough to adequately protect the IO""s of the chip. It would be beneficial to be able to detect over-voltage conditions which are not quite as high, such as a 0.1 volt over-voltage condition at pin in order to more fully protect IO devices from being damaged as a result of an over voltage condition at pin.
A general object of an embodiment of the present invention is to provide a method and apparatus for detecting a relatively small over-voltage condition at pin, such as a 0.1 volt over voltage, thereby preventing damage to IO devices due to over voltage at pin.
Another object of an embodiment of the present invention is to provide a method and apparatus where voltage levels are band-limited and source-followed, and a comparator is used to signal an over-voltage condition on the pin of an IO device.
Briefly, and in accordance with at least one of the foregoing objects, an embodiment of the present invention provides a method and circuit which provides that a source voltage and a pad voltage are band-limited and source-followed down in order to get them into the input range of a comparator, the output of which signals an over-voltage condition on the pad. The circuit provides the ability to provide the relationship between the source voltage and pad voltage to a comparator with a very small, tightly-controlled offset. This translates to the ability to detect very small over-voltage conditions on an IO. The circuit consumes little power, is highly accurate, and requires no special, expensive process options. The circuit can be used anywhere there is a desire to compare (with a small, accurate offset) two signals that are close to a source voltage, such as VDD. The circuit can also be used to compare signals close to VSS.